Semiconductor power switches are used in a variety of power applications, including, e.g., power conversion, power generation, and power transmission. Power switches are specifically designed to survive the steady-state conditions in such power applications. For example, power switches are designed to withstand relatively large potential differences between the main terminals (e.g., drain-to-source or collector-to-emitter voltages) in the OFF state and to conduct relatively large currents (e.g., drain or collector currents) in the ON state while dissipating relatively low power.
Power switches are also specifically designed to transition between a more conductive ON state and a less conductive OFF state in such power applications. Because of the relatively high currents and/or voltages in power applications, these transitions often bear an increased risk of failure for the power switch and the circuitry coupled thereto. For example, leads, wires, components, and circuitry that are coupled to a power switch often have a non-negligible intrinsic inductances. In the event of a large, rapid change in voltage—such as the voltage changes associated with the transition of a power switch between an ON state and an OFF state—such inductances can cause voltage spikes that are capable of harm.
In order to reduce the risk of harm associated with large, rapid changes in voltage, many semiconductor power switch drivers and controllers provide “soft” shutdown functionality. In particular, the potential on the control terminal (e.g., gate or base terminal) of a semiconductor switch is changed relatively slowly in order to reduce the rate of change of the current conducted by the power switch, as well as the rate of change of the potential difference across the main terminals of the power switch. Voltage spikes due to intrinsic inductances can be reduced or avoided.
Several approaches have been used to provide soft shutdown functionality for semiconductor switches. Many of these approaches can be treated as a variable-resistance element that is coupled in series with the control terminal of the power semiconductor switch. A closed loop control circuit is coupled to sense a short circuit or other overcurrent condition and adjust the resistance of the variable-resistance element in response. By adjusting this resistance, the magnitude of the drive signal that is coupled into the control terminal is adjusted to ensure that shutdown proceeds sufficiently slowly.
Often, short circuit or other overcurrent conditions are sensed using desaturation fault detection circuitry. As soon as a short circuit or other overcurrent condition occurs, the current between the main terminals increases very rapidly. The rapid increase in current between the main terminals may lead to a voltage across the main terminals that exceeds the expected voltage across those terminals at saturation. This high voltage can be sensed and used to trigger soft shutdown functionality.
Although soft shutdown functionality has been implemented successfully in certain operational conditions, the technical requirements for soft shutdown increase as the rate of switching and the magnitude of the switched voltages increase. For example, modern power modules—such as those implemented in part (e.g., hybrid modules, Si IGBT+SiC Schottky diodes) or in whole (e.g., SiC MOSFET+SiC Schottky diodes) with high-bandwidth semiconductor materials—can switch several 100's of kilowatts at frequencies in the range of 30-100 kHz or more. Despite the relatively low power losses and relatively high power densities provided by such modern power modules, even these power modules are able to endure short circuit and other overcurrent conditions for relatively short time periods. For example, the short circuit safe operating area (SCSOA) time of a modern power module may be a few microseconds. Further, avalanche operation may not be allowed.